Showing papers on "Noise (radio) published in 1995"

Time‐domain modeling of pulsed finite‐amplitude sound beams

Abstract: A time‐domain algorithm that solves the Khokhlov–Zabolotskaya–Kuznetsov (KZK) nonlinear parabolic wave equation is described. The algorithm models the propagation of pulsed finite amplitude sound beams radiated from axisymmetric sources in homogeneous, thermoviscous fluids. Numerical results are presented for waveform distortion and shock formation in directive beams radiated by pulsed circular pistons. Waveforms are calculated through the shock region and out to far‐field locations where they are dominated by the nonlinearly generated low‐frequency components. Effects of pulse duration, frequency modulation, and noise are examined. Methods for including relaxation and focusing are described.

Temporal correlation of electrons: Suppression of shot noise in a ballistic quantum point contact.

Abstract: Wideband shot noise, associated with dc current flow through a quantum point contact (QPC), is measured in the microwave frequency range of 8--18 GHz. As the number of conducting channels in the QPC changes the noise power oscillates. Consistent with existing theories, the noise peaks depend linearly on the dc current. Surprisingly, however, in the pinch off region, where QPC is expected to behave as a classical injector, we find strong noise suppression, possibly mediated by the Coulomb interaction.

Spatiotemporal stochastic resonance in excitable media.

Abstract: Pattern formation far from equilibrium has been studied very extensively in the last years (for a recent review, see [1,2]). Representative examples are Rayleigh-Benard convection rolls, Taylor-Couette flow, and spiral waves in the Belousov-Zhabotinsky reaction. Typically a pattern starts to build up when the control parameter (the temperature difference in case of the Rayleigh-Benard system) becomes larger than a critical value. Noise makes the bifurcation smooth by triggering the onset of the pattern even below threshold r , rc. The role of fluctuations for the onset and selection of patterns has been studied in some detail and is reported on in a number of articles in [3,4] and [5]. In this paper, we discuss the role of noise for the formation of patterns in two-dimensional excitable media from a different perspective. It has been shown that a certain amount of noise can amplify temporal patterns by increasing the system’s sen

The thermocoax cable as the microwave frequency filter for single electron circuits

Abstract: We have analyzed the possibility of using the resistive coaxial cable, Thermocoax(R) Philips as a microwave cryofilter for single electron experiments performed in a top‐loading system dilution refrigerator. The biasing and signal lines made of this cable are assumed to link a single electron device kept at the lowest temperature Td to the rf filter having a temperature Tf of several kelvin. The attenuation in a wide frequency range has been calculated and it turned out that a 40‐cm‐long piece of this cable with an outer diameter of 0.5 mm could drastically reduce the effect of noise from the 50 Ω source anchored at Tf (below 25 K), making its effect comparable with the effect of the thermal fluctuations in the device for Td≊40 mK. The calculated attenuation is in reasonable agreement with the data measured at frequencies up to 18 GHz.

On noise reduction in vertical seismic records below 2 mHz using local barometric pressure

Abstract: We propose a simple method for the reduction of noise in vertical seismic recordings at frequencies below 2 mHz. The method consists in subtracting the locally recorded, scaled, atmospheric pressure signal from the gravity record. The scale factor is frequency independent and can be linearly estimated from the data. While such a pressure correction was previously only used in the analysis of tidal records, it is equally efficient at the low-frequency end of the normal mode band and we use gravity recordings of the big 1994 Shikotan and Bolivian quakes to demonstrate this point. Our examples also show that this correction permits to achieve noise levels well below the New Low Noise Model (NLNM) [Peterson, 1993]. The success of the correction also sheds some light on the physical processes responsible for the generation of low-frequency seismic noise at quiet observatories.

White-Noise-Induced Transport in Periodic Structures

Abstract: Spatially periodic structures are exposed to additive Poissonian white shot noise of zero average. Because the underlying master equation no longer obeys the principle of detailed balance, these non-equilibrium fluctuations induce a macroscopic current—even in the absence of spatial asymmetry. The resulting current can be expressed in analytical closed form and we discuss its behaviour in the limits of very weak and very strong noise intensities. We find that the current increases monotonically and—in contrast to common intuition—saturates at infinitely strong noise intensity. The role of internal symmetries of the periodic structure is investigated from the viewpoint of optimizing the current amplitude at fixed noise intensity.

Design and Characterization of Two-Junction Tuning Circuits for Submillimeter SIS Mixers

Abstract: We report on the continuing development of submillimeter quasi-optical slot antenna SIS mixers which use two-junction tuning circuits [1]. The mixers use 10 kA/cm2 Nb/Al-Oxide/Nb junctions and Nb wiring, and have generated DSB receiver noise temperatures around 5hv/kB to 700 GHz, and 16hv/kB (620 K) at 798 GHz. We present Fourier-transform spectrometer (FTS) measurements and heterodyne measurements on several such devices. In general, the measured response at frequencies below the gap of Nb is in good agreement with the predicted performance. Introduction SIS mixers with Nb-trilayer tunnel junctions offer excellent performance at frequencies below 800 GHz, and should perform competitively at frequencies as high as 1.4 THz [2-5]. Our goal is to develop a suite of ultra-low-noise SIS mixers which cover frequencies from = 400 GHz to more than I THz. At frequencies below the Nb gap (< 700 GHz), existing devices perform quite well [6], but further improvements in noise temperatures remain important. In addition, it is quite useful to be able to predict device performance given the design parameters. One can then optimize the device design for a particular frequency band, and confirm that we indeed understand the physics of the device. Progress in the design and operation of devices in this frequency range is presented below. At frequencies above the Nb gap, RF photons can break Cooper pairs, resulting in greater signal loss and higher receiver noise temperatures. The loss is particularly important in the slender microstrip lines used to resonate the junction capacitance and to transform the antenna impedance down to the RF junction impedance. However, the actual coupling of radiation into Nb junctions above the gap should still be reasonably efficient -more than 30% at 800 GHz [7]. In addition to RF loss, one is also affected by increased dispersion at frequencies near the gap. The Nb microstriplines have therefore been carefully designed to ensure the desired bandpass is achieved. We demonstrate for the first time that all-Nb SIS receivers can have substantially better performance than GaAs Schottky receivers for the astronomically important CI ( P2 — P 1 ) and CO (7-6) transitions near 810 GHz. Receiver Configuration Since waveguide mixers become difficult to construct at short submillimeter wavelengths, we have adopted a quasi-optical approach. Here, lenses take the place of waveguide horns, and the incoming radiation is collected by a planar antenna on the SIS mixer substrate [8]. This offers several other advantages, such as on-chip broad-band silt a SS 114 SSE WA Wa Sta MX Dewar 120 Mylar Window SIS Mixer 295 IQ 77 K Mylar Load Bearnsplitter ICI 1= NZ IC =1 HEN1T Amplifier Polyethylene Lens

Tunneling and quantum noise in one-dimensional Luttinger liquids.

Abstract: We study non-equilibrium noise in the transmission current through barriers in 1-D Luttinger liquids and in the tunneling current between edges of fractional quantum Hall liquids. The distribution of tunneling events through narrow barriers can be described by a Coulomb gas lying in the time axis along a Keldysh (or non-equilibrium) contour. The charges tend to reorganize as a dipole gas, which we use to describe the tunneling statistics. Intra-dipole correlations contribute to the high-frequency ``Josephson'' noise, which has an algebraic singularity at $\omega=e^*V/\hbar$, whereas inter-dipole correlations are responsible for the low-frequency noise. Inter-dipole interactions give a $1/t^2$ correlation between the tunneling events that results in a $|\omega|$ singularity in the noise spectrum. We present a diagrammatic technique to calculate the correlations in perturbation theory, and show that contributions from terms of order higher than the dipole-dipole interaction should only affect the strength of the $|\omega|$ singularity, but its form should remain $\sim |\omega|$ to all orders in perturbation theory.

Relationship Between Unsteady Flow, Pressure Fluctuations, and Noise in a Centrifugal Pump—Part B: Effects of Blade-Tongue Interactions

Abstract: Maps of pressure distributions computed using PDV data, combined with noise and local pressure measurements, are used for identifying primary sources of noise in a centrifugal pump In the vicinity of the impeller pressure minima occur around the blade and near a vortex train generated as a result of non-uniform outflux from the impeller The pressure everywhere also varies depending on the orientation of the impeller relative to the tongue Noise peaks are generated when the pressure difference across the tongue is maximum, probably due to tongue oscillations, and when the wake impinges on the tip of the tongue

Simulation of the AII amacrine cell of mammalian retina: functional consequences of electrical coupling and regenerative membrane properties.

Abstract: The AII amacrine cell of mammalian retina collects signals from several hundred rods and is hypothesized to transmit quantal "single-photon" signals at scotopic (starlight) intensities. One problem for this theory is that the quantal signal from one rod when summed with noise from neighboring rods would be lost if some mechanism did not exist for removing the noise. Several features of the AII might together accomplish such a noise removal operation: The AII is interconnected into a syncytial network by gap junctions, suggesting a noise-averaging function, and a quantal signal from one rod appears in five AII cells due to anatomical divergence. Furthermore, the AII contains voltage-gated Na+ and K+ channels and fires slow action potentials in vitro, suggesting that it could selectively amplify quantal photon signals embedded in uncorrelated noise. To test this hypothesis, we simulated a square array of AII somas (Rm = 25,000 Ohm-cm2) interconnected by gap junctions using a compartmental model. Simulated noisy inputs to the AII produced noise (3.5 mV) uncorrelated between adjacent cells, and a gap junction conductance of 200 pS reduced the noise by a factor of 2.5, consistent with theory. Voltage-gated Na+ and K+ channels (Na+: 4 nS, K+: 0.4 nS) produced slow action potentials similar to those found in vitro in the presence of noise. For a narrow range of Na+ and coupling conductance, quantal photon events (approximately 5-10 mV) were amplified nonlinearly by subthreshold regenerative events in the presence of noise. A lower coupling conductance produced spurious action potentials, and a greater conductance reduced amplification. Since the presence of noise in the weakly coupled circuit readily initiates action potentials that tend to spread throughout the AII network, we speculate that this tendency might be controlled in a negative feedback loop by up-modulating coupling or other synaptic conductances in response to spiking activity.

Monte Carlo studies of x‐ray energy absorption and quantum noise in megavoltage transmission radiography

Abstract: The subject contrast of bony anatomy in megavoltage medicalradiographs is very low, making detection of bony landmarks difficult if additional noise sources are introduced into the images. One source of noise, which is inherent to the x‐ray detection process, is x‐ray energy absorption noise. X‐ray energy absorption noise results from variations in the amount of energy deposited in the imaging detector per interacting x ray. These variations increase the noise content of the image. In this study, EGS4 Monte Carlo simulations of x‐ray interactions in metal plate phosphor screen detectors have been performed to determine the distribution of energy absorption events within the phosphor screen. From these ‘‘absorbed energy distributions (AEDs)’’, the x‐ray energy absorption noise and the quantum absorption efficiency of the detector are determined. These calculations are performed for a range of detector thicknesses (0.1–4 mm) and x‐ray energies (0.1–10 MeV). A number of conclusions can be drawn from these investigations. (i) The x‐ray absorption noise reduces the detective quantum efficiency (DQE) of metal plate/phosphor screen detectors by as much as 50% at energies used in megavoltage imaging (1–10 MeV). (ii) It is important to include secondary particle (electron) transport in estimating the quantum absorption efficiency of these detectors. For instance, the quantum efficiency of a typical portal detector is approximately 2%, even though 4%–5% of the incident photons are attenuated. (iii) The metal ‘‘conversion’’ plate commonly used in megavoltage imaging enhances the DQE of the phosphor screen by increasing the quantum absorption efficiency and reducing the magnitude of the x‐ray absorption noise.

Earthquake site-response study in Giumri (formerly Leninakan), Armenia, using ambient noise observations

Abstract: We have tested whether ambient noise observations reveal useful information regarding site response at sediment sites in Giumri (formerly Leninakan), Armenia. This was done by comparing the noise observations with spectral-ratio site-response estimates based on aftershock data of the 7 December 1988 earthquake. We show that neither the horizontal-component noise spectra alone, nor spectral ratios taken with respect to a nearby bedrock site, reveal useful information regarding site response. However, spectral ratios of horizontal components taken with respect to vertical components (Nakamura's method) exhibit a prominent peak that corresponds to the fundamental resonant frequency observed in the aftershock data. Based on this and other recent investigations, we conclude that Nakamura's method may be a reliable procedure for determining the fundamental resonant frequency of sedimentary deposits from ambient noise observations.

Noise modeling of microwave heterojunction bipolar transistors

Abstract: Analytical expressions of microwave heterojunction bipolar transistors minimum noise figure and noise parameter are reported in this paper. These expressions are derived from a noise model including nonideal junctions, emitter and base resistances and have been compared with measured data obtained on a Si-SiGe HBT. An agreement between theoretical and experimental data was as observed up to 20 GHz for several bias conditions. The limits of the model or the range of validity of the proposed equations have been also examined with the help of an appropriate CAD software. The analysis of the influence of parasitic elements on noise parameters has shown a strong influence of the extrinsic base collector capacitance at microwave frequencies. >

Stochastic resonance in a tunnel diode in the presence of white or coloured noise

Abstract: We study the signal-to-noise ratio, signal and noise output levels in a fast bistable electronic system: a tunnel diode. We observe stochastic resonance when the system is driven by a sum of a small periodic signal and noise. The phenomenon is investigated for values of the driving frequency as high as 10 kHz. This is the highest frequency value used in SR experiments until now. In the presence of «white noise», we observe a nonmonotonic behavior characterized by a sharp dip in the output noise level measured at 100Hz and 1 kHz. A similar behavior is predicated by recent theories. We also present preliminary experimental results of SR in the presence of an Ornstein-Uhlenbeck noise. For the set of investigated values of the correlation time of the noise, we still observe the characteristic dip in the output noise levels. The height of the dip is affected by the correlation time of the noise.

Temporal Order in Dirty Driven Periodic Media.

Abstract: We consider the nonequilibrium steady states of a driven charge density wave in the presence of impurities and noise. In three dimensions at strong drive, a dynamical phase transition into a temporally periodic state with quasi-long-range translational order is predicted. In two dimensions, impurity induced phase slips are argued to destroy the "moving solid" phase. Implications for narrow band noise measurements and relevance to other driven periodic media, e.g., vortex lattices, are discussed.

Frequency and intensity noise in an injection-locked, solid-state laser

Abstract: We have calculated transfer functions for frequency and intensity fluctuations in an injection-locked solid-state laser. At modulation frequencies well below the locking frequency we find significant frequency-noise reduction, and at modulation frequencies above the locking frequency we find that the frequency noise is that of the free-running slave laser. Our intensity-noise theory predicts substantial damping of relaxation oscillations in the slave laser. To verify these results we have measured the frequency and intensity noise of a 5-W, injection-locked Nd:YAG laser.

Squeezing and intermode correlations in laser diodes.

Abstract: We demonstrate experimentally that the intensity noise of so-called "free-running single-mode" laser diodes results from a cancellation between very large anticorrelated fluctuations of the main mode, on one hand, and of many weak longitudinal side modes, on the other hand. When line narrowing techniques are used, intensity squeezing can be observed at room temperature, but this noise reduction is not always single-mode squeezing. These experimental results are in agreement with a simple phenomenological model using Langevin-type equations.

Noise controlled spiral growth in excitable media.

Abstract: We consider a two‐dimensional pulse‐coupled array of noisy threshold elements with a long‐range interaction. For tight coupling, we observe the formation of spatio‐temporal excitation waves, such as target and spiral waves. For weak coupling, we find noise sustained spiral growth, where the noise level controls the scale of the spiral.

Telecommunications speech signal improvement by reduction of residual noise

Abstract: A telecommunications network service overcomes the annoying effects of transmitted noise by signal processing which filters out the noise using a model-based iterative signal estimator. The estimator is provided with a current estimate of the noise power spectral density, using signal frame samples determined by a voice activity detector to be noise-only frames. The signal estimator makes intra-frame iterations of the current frame while using smoothing across LSP parameters of adjacent frames, recent past frames, and up to two contiguous future frames. Non-stationary noise created by the iterative filtering is further reduced in one or more post-filtering stages that use knowledge of the nature of the low level non-stationary noise events.

Quantum noise of free-running and externally-stabilized laser diodes

Abstract: We have investigated the intensity and phase noise of single-mode laser diodes, either free-running or using different types of line narrowing techniques at room temperature, namely feedback from an external grating and injection locking. We have measured an intensity squeezing of 1.2 dB in the first case, and 1.4 dB in the second case (respectively, 1.6 dB and 2.3 dB inferred at the laser output). We have observed that the intensity noise of a free-running 'single-mode' laser diode actually results from a cancellation effect between large anticorrelated fluctuations of the main mode and of weak longitudinal side modes. It is also shown that free-running diodes exhibit very large excess phase noise. Typically more than 80 dB above shot noise at 10 MHz, which can be significantly reduced by the above-mentioned techniques.

Magnetic resonance imaging apparatus

Abstract: In a magnetic resonance imaging apparatus a superconductive magnet is disposed in a first vacuum vessel defined by an outer cylinder and an inner cylinder. Another cylinder is coaxially disposed in the inner cylinder of the first vacuum vessel to form a second vacuum vessel. A gradient coil is disposed in the second vacuum vessel to prevent the noise from transmitting outside the second vacuum vessel.

Toward the direct calculation of noise - Fluid/acoustic coupled simulation

Abstract: A numerical technique for the direct calculation of flow generated noise is developed in this paper and applied to the prediction of supersonic jet noise. In this approach, each flow parameter is decomposed into a time-averaged mean and a time-dependent fluctuating part. The mean flow is established with the solution of the three-dimensional compressible Navier-Stokes equations in the first step. Flow perturbations based on the description of the large-scale structures as a linear superposition of normal mode instability waves are introduced at the nozzle exit plane. Their propagation in time and space are studied through solution of the Euler equations for the perturbations in the second step. Such an approach ensures that the fluctuation variables, which may be several orders of magnitude smaller than the mean values, are computed accurately without numerical round-off errors. Some dynamic features of the jet flow are presented. Predictions of radiated noise for a few test cases and qualitative comparisons with experiments are made. Effects of jet temperature on the peak directivity of the radiated sound are examined.

Low noise YBa2Cu3O7−x–SrTiO3–YBa2Cu3O7−x multilayers for improved superconducting magnetometers

Abstract: We have fabricated YBa2Cu3O7−x–SrTiO3–YBa2Cu3O7−x (YBCO–STO–YBCO) trilayers, in which each layer is patterned photolithographically, capping the first YBCO film with an in situ STO film. Atomic force microscopy demonstrates that the capping process dramatically improves the quality of the surface of the second layer, allowing the growth of an upper YBCO film with a substantially reduced level of low‐frequency flux noise. A magnetometer with a multiturn flux transformer coupled to a dc superconducting quantum interference device achieved a magnetic field noise of 74 fT Hz−1/2 at 1 Hz, improving to 31 fT Hz−1/2 at 1 kHz.

Escape from a fluctuating double well.

Abstract: We study thermally driven escape from a double well over a fluctuating barrier height. The fluctuations of the bistable potential are governed by exponentially correlated Gaussian noise of weak-to-moderate-to-large noise correlation time \ensuremath{\tau}. Exact results are obtained for the limiting cases of very fast (\ensuremath{\tau}\ensuremath{\rightarrow}0) and very slow (\ensuremath{\tau}\ensuremath{\rightarrow}\ensuremath{\infty}) barrier fluctuations. For finite noise color \ensuremath{\tau}, we present approximation schemes for the stochastic dynamics of nonlinear systems that are driven simultaneously by both a white noise source and a multiplicative colored noise (colored noise driven parametric stochastic flows). Our approximative results for arbitrary, but finite noise color \ensuremath{\tau} become exact for escape in a piecewise parabolic bistable potential with a cusp at the transition state.

Effect of Modification to Tongue and Impeller Geometry on Unsteady Flow, Pressure Fluctuations and Noise in a Centrifugal Pump

Abstract: Particle Image Velocimetry (PIV), pressure and noise measurements are used to study the effect of modifications to tongue and impeller geometries on the flow structure and resulting noise in a centrifugal pump. It is demonstrated that the primary sources of noise are associated with interactions of the non-uniform outflux from the impeller (jet/wake phenomenon) with the tongue. Consequently, significant reduction of noise is achieved by increasing the gap between the tongue and the impeller up to about 20% of the impeller radius. Further increase in the gap affects the performance adversely with minimal impact on the noise level. When the gap is narrow, the primary sources of noise are impingement of the wake on the tip of the tongue, and tongue oscillations when the pressure difference across it is high. At about 20% gap, the entire wake and its associated vorticity trains miss the tongue, and the only (quite weak) effect of nonuniform outflux is the impingement of the jet on the tongue. An attempt is also made to reduce the non-uniformity in outflux from the impeller by inserting short vanes between the blades. They cause reduction in the size of the original wakes, but generate an additional jet/wake phenomenon of their own. Both wakes are weak to a level that their impacts on local pressure fluctuations and noise are insignificant. The only remaining major contributor to noise is tongue oscillations. This effect is shown to be dependent on the stiffness of the tongue.Copyright © 1995 by ASME